Production of the E. coli LT-B enterotoxin subunit

ABSTRACT

Methods and compositions are provided for the cloning and expression of genes coding for the non-toxic subunit of the heat-labile enterotoxin (LT-B) of E. coli. The LT-B thus produced may be formulated for use as an immunogen in vaccines. Specific antibodies produced by this invention may be used in diagnostic tests for the detection of Vibrio cholerae or LT positive enterotoxigenic E. coli. The antibodies of this invention may further be formulated into passive vaccines for the prophylactic or therapeutic protection of human beings or other mammalian species against diarrheal diseases caused by Vibrio cholerae or by the LT positive enterotoxigenic E. coli or other bacteria.

This application is a continuation of application Ser. No. 07/936,426 filed Aug. 27, 1992 currently abandoned, which in turn is a continuation of application Ser. No. 06/846,173, filed Mar. 31, 1986 currently abandoned, which in turn is a continuation of application Ser. No. 06/628,873 filed Jul. 9, 1984 currently abandoned.

TABLE OF CONTENTS

1. Introduction

2. Background of the Invention

2.1. Enterotoxigenic Bacteria and Diarrheal Disease

2.2. Immunological Approaches to the Prevention and Cure of Enterotoxigenic Diarrheal Disease

2.3. Recombinant DNA Technology

3. Summary of the Invention

4. Brief Description of the Figures

5. Detailed Description of the Invention

5.1. Identification and Isolation of LT Genes

5.2. Identification and Isolation of LT-B Genes

5.3. Purification of LT-BNT

5.4. Preparation and Use of Antibodies Against LT-BNT

6. Example

6.1. General Procedures for Recombinant Plasmid Preparation

6.1.1. Conditions for Restriction Enzyme Digestions

6.1.2. Purification of DNA Digestion Products

6.1.3. T4 DNA Ligation

6.1.4. Transformation and Isolation of Recombinants

6.2. Methods for LT Gene Production Analysis

6.2.1. Enzyme-Linked Immunosorbent Assay (ELISA)

6.2.2. Yl Adrenal Cell Enterotoxin Assay

6.2.3. Rat Ileal Loop Assay of Enterotoxin Activity

6.3. Preparation and Isolation of Specific LT-B-Producing Clones

6.3.1. Isolation of pDF82

6.3.2. Isolation of pDF87

6.3.3. Isolation of pJC217

6.4. Recovery of LT-BNT

6.4.1. Growth of E. coli K-12 Containing the pJC217 Plasmid

6.4.2. Cell Disruption

6.4.3. Affinity Chromatographic Purification of LT-BNT

6.5. Analysis of LT-BNT for Toxicity

6.6. Preparation of Antiserum Against LT-BNT

6.6.1. Preparation of Antiserum Against LT-BNT in Mice

6.6.2. Preparation of Antiserum Against LT-BNT in a Goat

6.6.3. Production of Monoclonal Antibodies

6.7. Neutralization of Enterotoxin Activity by Antiserum Against LT-BNT

6.8. Analytical Systems Based on LT-BNT

6.8.1. ELISA Assay

6.8.2. Immunoblot Analysis

6.8.3. Latex Bead Agglutination

6.9 Deposit of Microorganism

1. INTRODUCTION

This invention relates to a process for the production of a non-toxic B subunit of the heat-labile enterotoxin (LT-B) from a human isolate of an enterotoxigenic strain of Escherichia coli. This process utilizes recombinant DNA techniques, in which the requisite gene sequence is inserted by means of a suitable DNA vector into a non-pathogenic microbial strain.

The present invention further relates to the isolation and purification of the LT-B protein and to its use for immunological prophylaxis and therapy. Antiserum produced against the LT-B protein may be utilized for passive immunization against enterotoxin-induced diarrheal disease. Alternatively, such antiserum may be employed for the preparation of immunological diagnostic tests that are specific for the enterotoxins of Escherichia coli or of Vibrio cholerae.

2. BACKGROUND OF THE INVENTION 2.1. ENTEROTOXIGENIC BACTERIA AND DIARRHEAL DISEASE

Acute diarrheal disease due to the temporary colonization of the small intestine by enterotoxigenic strains of certain bacteria is a major health problem of global significance. Among the responsible bacteria, perhaps the most widely recognized is Vibrio cholerae. Less well known but of greater practical significance are particular strains of Escherichia coli (E. coli) which, together with rotavirus, produce acute diarrheic episodes that are fatal each year to an estimated 10 million infants living in underdeveloped tropical countries [Black et al., Lancet i: 141 (1981)].

These E. coli strains also generally account for a high incidence of the acute diarrhea that afflicts visitors to tropical regions. Furthermore, they have a profound impact upon livestock as well, since Kohler [J. Am. Vet. Med. Assoc. 173: 588 (1978)] has reported that weanling animals, particularly calves, lambs and piglets, may be similarly affected.

Both Vibrio cholerae and the enterotoxigenic E. coli strains produce their diarrheic effects through production of an enterotoxin. The cholera enterotoxin has been isolated and purified to homogeneity by Finkelstein [Crit. Rev. Microbiol. 2: 553 (1973)]. Furthermore, Finkelstein and LoSpalluto [J. Exp. Med. 130: 185 (1969)] have separated a protein subunit from the cholera toxin that has reduced biological activity. What has emerged from these and from other studies is the finding that the cholera enterotoxin is an 84,000 dalton protein that consists of an A and a B subunit.

The A subunit (28,000 daltons) is responsible for the biological effects of the toxin but is incapable of binding to its target receptors alone. Through the action of sulfhydryl reagents, the A subunit may be cleaved into two polypeptide chains, with molecular sizes of 7,000 and 21,000 daltons. Of these chains only the larger, designated A₁, is active.

The B subunit, which has a size of 56,000 daltons, is essential for the expression of the activity of the A subunit. Apparently it acts by binding to a target cell and then facilitating penetration by the active A subunit. Finkelstein et al. J. Immunol. 113: 145 (1974)] have that the B subunit consists of non-covalently associated subunits that can be dissociated by vigorous treatment with sodium dodecyl sulfate or at low pH with urea into five polypeptide chains.

The effects of cholera toxin have been demonstrated by Sheer et al. Gastroenterology 65: 895 (1973)] in rabbit jejunum. In that system, the toxin causes a blood to lumen unidirectional flux of sodium. As a result, the intestinal fluid becomes low in protein, Mg⁺⁺ and high in K⁺, Na⁺ and HCO₃ ⁻, compared to normal serum levels. With these ionic changes, there is a concomitant outflowing of water to the lumen, for the maintenance of osmotic equilibrium with the blood plasma.

The precise structure of the cholera toxin receptor is unknown, but it appears to be a glycolipid. This observation is based upon a finding by King and van Heyningen [J. Infect. Dis. 131: 643 (1975)] that the binding of cholera toxin to membranes is inhibited by various glycosphingolipids. Of the compounds of this type examined, G_(M1) (galactosyl-N-acetylgalactosaminyl-examined, (sialyl)-galactosylglucosylceramide) was most potent.

Once cholera toxin binding occurs, there is a stimulation of adenylate cyclase activity and a locking of that enzyme in the activated state. The result is an increase in intracellular levels of cAMP that in some way gives rise to the above ionic changes.

Enterotoxic strains of E. coli also mediate their diarrheic effects through the production of enterotoxins. These toxins are of two types, one of which is a relatively low molecular weight species of 2,000 daltons. Because it survives treatment at 100° C., this species is referred to as the heat-stable toxin (ST). A second toxin that is heat labile (LT) is remarkably similar to the cholera toxin.

As shown by Gill et al. [Infect. Immun. 33: 677 (1981)], E. coli LT consists of the same type and number of subunits as the cholera toxin, and the corresponding subunits have approximately the same molecular weights. As with cholera toxin, the B subunits of LT attach to intestinal mucosal glycolipid receptors, thus permitting penetration of the cell by the biologically active A subunit. The sequence of events from that point on is also similar. Most importantly, Clements and Finkelstein [Infect. Immun. 21: 1036 (1978)] have shown that E. coli LT is immunologically related to both the A and B subunits of cholera enterotoxin.

2.2. IMMUNOLOGICAL APPROACHES TO THE PREVENTION AND CURE OF ENTEROTOXIGENIC DIARRHEAL DISEASE

The most practical means for combating the widespread morbidity and mortality caused by microbial toxin-induced diarrheal disease would be protective vaccination. In the case of the enterotoxigenic E. coli strains, three approaches might be taken.

First, somatic antigens could be used for immunization. Killed or attenuated bacteria could be employed for this purpose, but this approach entails some risk and is likely to be of limited effectiveness If the cell killing or attenuation is incomplete, clinical disease may develop. Even if this does not occur, protection will be imperfect since antigenically dissimilar somatic serotypes will not be recognized.

Secondly, Acres et al. Infect. [Immun. 25: 121 (1979)] have shown that pilus-mediated anchorage is a prerequisite for the induction of diarrheal disease by certain strains of enterotoxin-secreting E. coli. Thus, interference with cellular adhesion would have a prophylactic effect. Such interference could be produced by vaccination with pilus antigens, but again any protection so conferred would be applicable only to antigenically similar bacteria. Morgan et al. [Infect. Immun. 22: 771 (1978)] have detected multiple antigenically dissimilar pilus antigens among animal and human enterotoxigenic E. coli strains.

Finally, it should be possible to vaccinate animals with the enterotoxin itself. The immunity thus established would provide protection against active challenge with any of the relevant E. coli strains that produce the toxin. For reasons not clearly understood, immunization with LT toxin appears to provide protection against strains producing both LT and ST. There would not be protection against strains that produce only ST, but these strains are in the minority. Klipstein and Engert [Infect. Immun. 23:592 (1979)] have described the active immunization of rats with purified LT protein.

Although immunization may be achieved through the use of LT itself, the use of the biologically inactive B subunit (LT-B) alone should be almost as effective, and of course safer. The efficacy of this approach has been shown in rats by Klipstein and Engert [Infect. Immun. 31:144 (1981)]. Such immunization should also confer protection against cholera-induced diarrheic attacks, because of the immunological relationship between LT and the cholera enterotoxin described above.

Klipstein et al. have also immunized rats with ST coupled to LT [Infect. Immun. 32:1100 (1981)] or to the LT-B protein [J. Infect. Disease 147:318 (1983)]. A patent based on such conjugates and their use as vaccines has been issued to Klipstein et al. [U.S. Pat. No. 4,411,888].

2.3. RECOMBINANT DNA TECHNOLOGY

In current recombinant DNA procedures, specific DNA sequences are inserted into an appropriate DNA vehicle, or vector, to form recombinant DNA molecules that can replicate in host cells. Circular double-stranded DNA molecules called plasmids are frequently used as vectors, and the preparation of such recombinant DNA forms entails the use of restriction endonuclease enzymes that can cleave DNA at specific base sequence sites. Once cuts have been made by a restriction enzyme in a plasmid and in the segment of foreign DNA that is to be inserted, the two DNA molecules may be covalently linked by an enzyme known as a ligase. General methods for the preparation of such recombinant DNA molecules have been described by Cohen and Boyer in U.S. Pat. No. 4,237,224. Other useful general methods have been described by Collins and Hohn in U.S. Pat. No. 4,304,863. Because of their broad utility, these patents are hereby incorporated by reference.

Once prepared, recombinant DNA molecules can be used to produce the product specified by the inserted gene sequence only if a number of conditions are met. Foremost is the requirement that the recombinant molecule be compatible with, and thus capable of autonomous replication in, the host cell. Much recent work has utilized Escherichia coli (E. coli) as a host organism because it is compatible with a wide range of recombinant plasmids. Depending upon the vector/host cell system used, the recombinant DNA molecule is introduced into the host by transformation, transduction or transfection.

Detection of the presence of recombinant plasmids in host cells may be conveniently achieved through the use of plasmid marker activities, such as antibiotic resistance. Thus, a host bearing a plasmid coding for the production of an ampicillin-degrading enzyme could be selected from unaltered cells by growing the host in a medium containing ampicillin. Further advantage may be taken of antibiotic resistance markers where a plasmid codes for a second antibiotic-degrading activity, at a site where the selected restriction endonuclease makes its cut and the foreign gene sequence is inserted. Host cells containing properly recombinant plasmids will then be characterized by resistance to the first antibiotic but sensitivity to the second.

The mere insertion of a recombinant plasmid into a host cell and the isolation of the modified host will not in itself assure that significant amounts of the desired gene product will be produced. For this to occur, the foreign gene sequence must be fused in proper relationship to a signal region in the plasmid for DNA transcription called a promoter. Alternatively, the foreign DNA may carry with it its own promoter, as long as it is recognized by the host. Whatever its origin, the promoter is a DNA sequence that directs the binding of RNA polymerase and therefore "promotes" the transcription of DNA to messenger RNA (mRNA).

Given strong promotion that can provide large quantities of mRNA, the ultimate production of the desired gene product will be dependent upon the effectiveness of translation from mRNA to protein. This, in turn, is dependent upon the efficiency of ribosomal binding to the mRNA. In E. coli, the ribosome-binding site on mRNA includes an initiation codon (AUG) and an upstream Shine-Dalgarno (SD) sequence. This sequence, containing 3-9 nucleotides and located 3-11 nucleotides from the AUG codon, is complementary to the 3' end of E. coli 16S ribosomal RNA (rRNA) [Shine and Dalgarno, Nature 254: 34 (1975)]. Apparently, ribosomal binding to mRNA is facilitated by base pairing between the SD sequence in the mRNA and the sequence at the 16S rRNA 3' end. For a review on maximizing gene expression, see Roberts and Lauer, Methods in Enzymology 68: 473 (1979).

The introduction of LT plasmids from enterotoxigenic E. coli strains of human and porcine origin into other bacteria has recently been demonstrated by Neill et al. [Infect. Immun. 41:1056 (1983)]. In that study, LT plasmids from E. coli were transferred by conjugation into E. coli K-12 strains and into strains of Shigella flexneria, Shigella sonnei, Citrobacter freundii, Enterobacter cloacae, Klebsiella pneumoniae and Salmonella typhimurium. Analysis of the transconjugants showed that in all cases the transferred plasmids were stably maintained in their hosts. LT gene expression measured by solid-phase radioimmunoassay varied widely, however, with maximal LT production occurring in E. coli.

Genetic engineering techniques can also be applied to produce the B subunit of LT. Dallas [European Patent Application Serial No. 0060129] has described the cloning of the gene coding for LT-B from an E. coli isolate of porcine origin. The cistron encoding the B subunit of LT was cloned into vector pJJS500 by cleaving EWD299 with EcoRI and ligating this DNA to EcoRI cleaved pJJS500. The application stated that a plasmid specifying LT-B production without apparent LT-A contamination was thus obtained. It should be pointed out that no evidence was presented to support this claim, no mention of in vivo or in vitro studies, and there was no indication of successful antibody production, based on the gene product.

Yamamoto et al. [J. Bacteriol. 148:983 (1981)] have described the cloning of the LT-B gene from a human E. coli isolate into plasmid pBR322. Some expression of the gene product was detected by growing the modified bacterium in a radiolabeled amino acid mixture and then analyzing crude cell lysates by SDS polyacrylamide gel electrophoresis. No effort was made, however, to purify or characterize the LT-B protein, and it is not known whether the level of gene expression was significant.

In other studies, Sanchez et al. [FEMS Microbiol. Lett. 14:1 (1982)] cloned the LT-B gene from a human isolate into vector pACYC184 Again, the gene product was neither purified nor characterized.

3. SUMMARY OF THE INVENTION

Methods and compositions are provided for the cloning and expression in single-cell host organisms of genes coding for the non-toxic subunit of the heat-labile enterotoxin (LT-B) of an enterotoxigenic E. coli strain Also described are methods for the selection and culturing of the modified hosts to produce LT-B, and for the isolation and purification of this product.

The LT-B thus produced may be utilized by the methods of this invention for a number of important immunological processes. It may be formulated for the production of vaccines having utility in veterinary and human medicine. Through passive administration, the antibodies from such vaccines may be used for the prevention and/or treatment of cholera-like enterotoxin-induced diarrheal disease in human beings and in other mammalian species. Through utilization in immunological diagnostic systems, the same antisera may be applied to the detection of cholera-like enterotoxins. As used in the present application, the term "chloera-like enterotoxin" shall mean cholera toxin and LT, as well as immunologically related enterotoxins naturally produced by E. coli, Vibrio cholerae or other gram-negative enteric bacilli, or produced by expression of the gene encoding cholera toxin, LT or such related enterotoxin in any microorganism, including strains of Salmonella, Yersinia, Pseudomonas, Shigella, Citrobacter, Klebsiella, and the like.

Unlike all other LT-B proteins that have been examined, whether derived from the chemical separation of the complete LT enterotoxin into its A and B subunits or from gene cloning, the product of this invention is non-toxic. This unusual freedom from toxic effects renders this invention uniquely suited for use in immunization procedures. To distinguish the LT-B of this invention from the toxic forms produced by other methods, it is designated LT-B non-toxic (LT-BNT).

4. BRIEF DESCRIPTION OF THE FIGURES

The present invention may be more readily understood by reference to the following figures (not drawn to scale), wherein

FIG. 1 is a schematic representation of the production of pDF82, an LT expression plasmid derived from pBR322, into which a 5.2 kb fragment containing the LT gene from an Ent plasmid of a human isolate of E. coli has been inserted;

FIG. 2 is a schematic representation of the construction of plasmid pDF87, through the insertion into the single HindIII site of pBR322 of a 0.8 kb fragment containing the LT-B gene and derived from plasmid pDF82 by HindIII excision; and

FIG. 3 is a schematic representation of the production of plasmid pJC217 by insertion into the single HindIII site of pUC8 of a 0.8 kb fragment containing the LT-B gene and derived from plasmid pDF87.

5. DETAILED DESCRIPTION OF THE INVENTION

This invention relates to the use of gene splicing methodology to produce a completely non-toxic biologically inactive subunit of the enterotoxin of an enterotoxigenic bacterial strain. This invention further relates to the use of the purified subunit as an immunogen for the production of polyvalent antiserum. Such antiserum has applicability to the prevention and cure in human beings or in other mammalian species of diarrheal disease that has as its origin enteric infection by strains of E. coli, Vibrio cholerae or other bacteria producing a cholera-like enterotoxin. The antiserum is also useful for the preparation of diagnostic tests for the presence of the cholera-like enterotoxins of these microorganisms.

For the purpose of illustration, the procedures of this invention are detailed using one particular enterotoxigenic strain of E. coli as an example. The fact that this microorganism was a human isolate may lead to a more potent antiserum for use in human beings. It must be emphasized, however, that there is strong cross reactivity between the comparable subunits of the toxins of many enterotoxigenic strains, whether they be of human, porcine or other origin. Thus this invention contemplates the potential use of any of them for this purpose, and the methods described herein are equally applicable to them all.

The methods of this invention entail a number of steps which, in logical sequence, include (1) identification and isolation of the gene encoding LT-B or a fragment thereof, (2) insertion of this gene or gene fragment into an appropriate cloning vehicle, (3) transfer of the genetically altered cloning vehicle into a compatible and single-cell host organism, (4) selection and growth of properly modified hosts that can replicate express the inserted gene sequences, (5) identification and purification of the gene product, (6) use of the gene product for antibody production, and (7) use of the specific antibodies for therapeutic, prophylactic and diagnostic purposes.

5.1. IDENTIFICATION AND ISOLATION OF LT GENES

The genes for the production of LT and its subunits are carried on plasmids (Ent plasmids) of enterotoxigenic E. coli strains. Thus a stool sample from a human being or other mammalian species afflicted with enterotoxin-induced diarrheic disease could serve as the crude source of the requisite gene sequences. Isolates from these sources may be grown in sufficient quantities using standard microbiological techniques that are well known to skilled practitioners in the art. Unfortunately, the ability to make enterotoxin confers no selective value upon the strains of E. coli that carry the Ent plasmid and produce enterotoxin. To monitor the transfer of the Ent plasmid into a stable laboratory strain such as E. coli K-12, a desirable first step, it is thus necessary to mark the plasmid in some way.

In the illustrative embodiment of the present invention, the plasmids of a human isolate of E. coli H10407 were phenotypically tagged by transposition from an F'tslac :: Tn5 plasmid as described by Sansonetti et al. Infect. Immun. 34:75 (1981)]. The tagged plasmids were then transferred by conjugation to K-12 strain 711, and an LT-producing transconjugant was selected. This transconjugant contained two large plasmids of a size (6×10⁷ daltons) that Gyles et al. [J. Infect. Dis. 130:40 (1974)] had shown to be characteristic of plasmids producing enterotoxin in H10407.

Verification of the fact that the transconjugant produced LT was made by enzyme linked immunosorbent assay (ELISA), using antibodies produced against LT, and by biologic activity as determined by induction of morphologic alterations in cultured mouse Y-1 adrenal cells. The plasmids thus transferred were isolated by the cleared lysate technique of Bolivar and Backman [Methods in Enzymology 68:245-267 (1979)], and the specific LT gene sequences were isolated by restriction endonuclease cleavage.

In the illustrative embodiment, the purified ENT plasmid was cut with the restriction endonuclease Pst I, although any restriction enzyme or combination thereof could be employed so long as LT production (and subsequently LT-B production) is not destroyed by excision in the critical gene regions. The particular enzyme chosen would preferably be one that makes a single cut in the cloning vehicle used. Fulfillment of this second requirement may easily be achieved, since detailed restriction maps of many of the commonly used cloning vehicles are available.

Once appropriate cuts were made by Pst I in both the ENT plasmid and in the cloning vehicle, in this example plasmid pBR322, the LT gene fragment was ligated to the cloning vehicle by use of an appropriate ligating enzyme. Representative of ligating enzymes are the DNA ligases from E. coli and from bacteriophage T4. Such enzymes form new phosphodiester linkages in conjunction with ATP or NAD⁺ as a cofactor.

Transformation of host bacterial cells with these recombinant DNA molecules containing the LT DNA fragments provides for the generation of copies of the requisite DNA, which can then be analyzed for production of LT as described above or used as a source of plasmid DNA for the subsequent isolation of specific gene fragments coding for production of LT-B only.

The insertion of the LT DNA restriction fragment into a cloning vector is easily accomplished when both the ENT plasmid and the described cloning vehicle have been cut with the same restriction enzyme, since complementary DNA termini are thereby produced. If this cannot be accomplished, it may be necessary to modify the cut ends that are produced by digesting back single-stranded DNA to produce blunt ends, or by achieving the same result by filling in the single-stranded termini with an appropriate DNA polymerase. In this way, blunt end ligation with an enzyme such as T4 ligase may be carried out. Alternatively, any site desired may be produced by ligating nucleotide sequences (linkers) onto the DNA termini. Such linkers may comprise specific oligonucleotide sequences that encode restriction site recognition sequences. The cleaved vector and the LT DNA fragment may also be modified by homopolymeric tailing, as described by Morrow [Methods in Enzymology 68:3 (1979)].

Alternatives to the isolation of the LT gene, or fragments thereof, include but are not limited to the chemical synthesis of the gene sequences (if known) or the preparation of DNA that is complementary to the messenger DNA that encodes the LT gene.

5.2. IDENTIFICATION AND ISOLATION OF LT-B GENES

The gene fragment that codes for production of LT-B is adjacent to and downstream from the gene fragment that codes for production of LT-A. There are a number of restriction endonuclease sites within the LT gene that flank these specific gene fragments. Thus, a restriction enzyme is chosen which does not cut into the structural gene for the fragment under study (LT-B)

Once identified and isolated, the LT-B gene or gene fragment is inserted into an appropriate expression vector, which contains the necessary elements for transcription and translation of the inserted gene sequences. Since the promoter region for the LT-B subunit is normally furnished by the A subunit gene and this toxic product is to be avoided, the expression vehicle must contain its own promoter which can be read in sequence with the LT-B gene. Many plasmids into which the LT-B gene may be inserted contain such promoter regions, the tetracylcine resistance gene of plasmid pBR322 and the lac gene of plasmid pUC8 being only two examples.

The efficient transcription of the LT-B gene or gene fragment is further dependent upon the presence of specific initiation signals. One signal that is commonly employed is the ATG sequence Sources of the ATG sequence include but are not limited to the cro or N genes of coliphage lambda and the E. coli tryptophan E, D, C, B or A genes. Such initiation sequences may be found among many other gene sequences into which the LT-B gene or fragment may be inserted, and they may be produced synthetically in the alternative.

Strong efficient translation is tied to the availability of a Shine-Dalgarno (SD) sequence which facilitates ribosomal attachment. Such SD sequences must be interposed between the promoter and the initiation signal, for efficient message readout. The objective high level production of the LT-B protein is thus dependent upon the insertion of the LT-B gene sequence downstream from promoter, SD and initiation sequences

Numerous cloning vehicles meeting these requirements may be employed which include but are not limited to SV40, adenovirus, yeast, lambda gt-WES-lambda B Charon 4A and 28, lambda-gt-l-lambda B, M13-derived vectors such as pUC8, 9, 18 and 19, pBR313, 322 and 325, pAC105, pVA51, pACY177, pKH47, pACYC184, pUB110, pMB9, ColE1, pSC101, pML21, RSF2124, pCR1 or RP4.

Many of these cloning vehicles contain one or more marker activities that may be used to select for desired transformants, such as ampicillin and tetracycline β-galactosidase activity in pUC8. Selection is greatly simplified when the host cells into which such vectors are inserted contain none of these activities, and one of the activities is lost by virtue of the insertion of the LT-B gene or gene fragment

The transfer of the recombinant cloning vector into the host cell may be carried out in a variety of ways. Depending upon the particular vector/host cell system chosen, such transfer may be effected by transformation, transduction or transfection

Depending upon the quality and quantity of the LT-B subunit produced, one or more clones may have to be prepared. In the illustrative embodiment of this invention, it was necessary to transfer the LT-B gene successively into two plasmids pBR322 and finally into plasmid pUC8. This multiple cloning sequence was necessitated by the fact that while all clones produced LT-B protein, the pBR322 recombinants produced low levels of LT-B (c.a. 1 mg/liter) and yielded toxic gene products. This toxicity was manifested by analysis in the Yl adrenal cell assay system, which is described in detail in Section 6.2.2., infra. The basis of this toxicity was not understood, although the toxic LT-A subunit was not detectable in the preparation.

In the specific embodiment described herein, final LT-BNT production at levels 50 fold higher than present in wild type enterotoxigenic E. coli strains was achieved.

5.3. PURIFICATION OF LT-BNT

As produced in E. coli K-12, LT-BNT remains in the periplasmic space. To free the desired subunit product of this invention it is thus necessary to disrupt the outer membrane. This is preferably accomplished by sonication, or by other mechanically disruptive means, such as the French pressure cell.

Cell disruption could also be accomplished by chemical or enzymatic means. Since divalent cations are often required for cell membrane integrity, treatment with appropriate chelating agents such as EDTA or EGTA might prove sufficiently disruptive to facilitate the leakage of LT-BNT from the cells. Similarly, enzymes such as lysozyme have been used to achieve the same result with proteins other than LT-BNT. That enzyme hydrolyzes the peptidoglycan backbone of the cell wall. In the specific illustration of the invention described below, however, lysozyme caused a 60 percent loss of recoverable LT-BNT.

The application of osmotic shock could also be employed. Briefly, this could be accomplished by first placing the cells in a hypertonic solution which would cause them to lose water and shrink. Subsequent, placement in a hypotonic "shock" solution would then lead to a rapid influx of water into the cells with an expulsion of the desired LT-BNT.

Once freed from the cells, LT-BNT may be concentrated by precipitation with salts such as sodium or ammonium sulfate, ultrafiltration or other methods well known to those skilled in the art. Further purification could be accomplished by conventional protein purification techniques including but not limited to gel filtration, ion-exchange chromatrography, preparative disc-gel or curtain electrophoresis, isoelectric focusing, low temperature organic solvent fractionation, or countercurrent distribution. Purification is preferably carried out, however, by the exploitation of a peculiar property of LT, LT-B and LT-BNT--an affinity for binding to agarose.

Both the complete toxin and the B subunit bind tenaciously to the galaclosyl residues of agarose. Thus LT-BNT is best purified by the selective retention of the subunit following the passage of a solution containing LT-BNT through an agarose column. Once bound and purged of other proteins by washing the column with buffer, the subunit may be freed by passing a galactose-containing solution through the column. This affinity chromatographic technique works well with E. coli K-12 because it is a rough bacterial strain. Wild-type strains bind the LT-BNT produced to galactosyl residues in their outer membranes, and very little of the subunit can be recovered on agarose columns from these strains. Thus, while the technique has occasionally been successfully employed with wild-type strains, best results are obtained with E. coli K-12 into which the LT-BNT genes have been inserted.

5.4. PREPARATION AND USE OF ANTIBODIES AGAINST LT-B

One purpose of this invention is the production of the non-toxic subunit of the heat-labile enterotoxin of an enterotoxigenic E. coli bacterium, by recombinant DNA techniques. Such subunit may then be employed as an immunogen in a vaccine to produce a protective immunological response against cholera-like bacterial-induced diarrheic infections in human beings or in animals. Because the LT-BNT subunit is antigenically related to the heat-labile enterotoxins of all of the enterotoxigenic E. coli strains and of Vibrio cholerae, such vaccination would provide wide immunity. The fact that the product of this invention is biologically inactive and completely non-toxic ensures that it can be used with a degree of safety that is unattainable with complete toxins or microorganisms, even if the latter are killed or attenuated.

After purification as described above in Section 5.3, the isolated LT-BNT subunit may be used directly as a vaccine or incorporated at an appropriate concentration into an adjuvant for vaccination. Such appropriate concentrations are known to one of skill in this field or are able to be determined by routine experimentation.

Suitable adjuvants for the vaccination of animals include but are not limited to Freund's complete or incomplete adjuvant (not suitable for human or livestock use), Adjuvant 65 (containing peanut oil, mannide monooleate and aluminum monostearate), and mineral gels such as aluninum hydroxide, aluminum phosphate and alum; surfactants such as hexadecylamine, octadecylamine, lysolecithin, dimethyldioctadecylammonium bromide, N₁ -N-dioctadecyl-N'-N-bis(2-hydroxyethyl-propanediamine), methoxyhexadecylglycerol, and pluronic polyols; polyanions such as pyran, dextran sulfate, poly IC, polyacrylic acid, carbopol; peptides such as muramyl dipeptide, dimethylglycine, tuftsin; and oil emulsions. LT-BNT could also be administered following incorporation into liposomes or other microcarriers, or after conjugation to polysaccharides, other proteins or other polymers.

The illustrative embodiment of this invention proved so immunogenic that high antibody titers were obtained in mice without the use of an adjuvant. [See Section 6.6.1, infra.]

Through active immunization in this fashion, the protection of livestock, other domestic animals and human beings can be achieved. Such protection would depend primarily upon the production of an adequate secretary IgA response. For this reason, and because human and other mammalian neonates are relatively immunologically incompetent when most susceptible to diarrheic infection, it is preferable that a passive immunization approach be taken. Accordingly, anti-LT-BNT antiserum could be produced in a large mammalian species such as a goat, cow, steer, sheep, horse or in avian species, and the IgG fraction isolated through plasmaphoresis or other means. This fraction could then be administered to human infants through a suitable carrier or infant food, such as infant formula or cereal. For livestock, the immune globulins could be given after incorporation into livestock feed or a suitable pharmaceutical vehicle. Livestock of particular interest are newborn piglets, calves and lambs.

The immune globulins of this invention can be combined with either a liquid or solid pharmaceutical carrier, and the compositions can be in the form of tablets, capsules, powders, granules, suspensions or solutions. The compositions can also contain suitable preservatives, coloring and flavoring agents, or agents that produce slow release. Potential carriers that can be used in the preparation of the pharmaceutical compositions of this invention include, but are not limited to, gelatin capsules, sugars, cellulose derivatives such as sodium carboxymethyl cellulose, gelatin, talc, magnesium stearate, vegetable oil such as peanut oil, etc. glycerin, sorbitol, agar and water. Carriers may also serve as a binder to facilitate tabletting of the compositions for convenient oral administration.

Of course, monoclonal antibodies could be produced through current technology to achieve the same result. Somatic cells with the ability to produce antibodies, such as B cells, may be fused with B-cell myeloma line cells to produce hybridoma cells. These cells may be cultured in vitro or as ascites tumors indefinitely to produce large quantities of specific antibodies. Because hybridoma cells may be readily cloned, it is possible to rapidly produce large numbers of cells, all of which produce the same specific antibody molecules directed to a common antigenic determinant. This exceptional uniformity in antibody production may be advantageous where the antibodies are to be used in specific diagnostic tests.

Lymph nodes and spleens of animals of animals primed by injection of an antigen are convenient sources of B cells, although it is equally feasible to remove these cells from unsensitized animals, and to prime them in vitro after isolation. Mouse and rat B lymphocytes are most frequently used in hybridoma production, but cells from rabbits, human beings, frogs or other animals might be used instead In the preferred embodiment of this invention, mouse spleen cells sensitized to LT-BNT in vitro are used to make the fused cell hybrids.

Numerous specialized myeloma cell lines have been developed from lymphocytic tumors for use in hybridoma production [Kohler and Milstein, Europe. J Immunol. 6: 511-519 (1976); Shulman et al., Nature 276:269-270 (1978)]. Of the many such cell lines produced, P3/X63-Ag 8, P3/NSI/1-Ag 4-1, Sp2/0-Ag14, and S194/5.XX0.BU.1 have frequently been used. In the example of the present invention, a murine myeloma cell line designated X63-Ag 8.653 is preferred.

The fusion of antibody-producing spleen or lymph node cells with myeloma cells to produce hybridomas is usually carried out with an excess of splenocytes or lymphocytes over myeloma cells that may be as high as 20:1 although, typically, lower ratios are used. Fusion is facilitated by the use of a fusion-promoting agent such as UV-inactivated Sendai virus or polyethylene glycol (PEG). Gefter et al. [Somatic Cell Genet. 3:231-236 (1977)] have reported that combining dimethyl sulfoxide with PEG further enhances cell fusion. Electrical devices are also available which can fuse cells with an exceptionally high degree of efficiency.

Once fusion has occurred, the hybridoma cells must be selected from the unfused parental cell strains. This selection process may be readily accomplished by culturing the cells in a medium that supports hybridoma but not parental cell growth. The somatic B cells used in the fusion have limited lifespans in culture and thus will be lost as they undergo sensecence and death, but the parental myeloma cells, with indefinite culture lifespans, must be eliminated by special selection techniques

In the example of the present invention, myeloma cells lacking hypoxanthine phosphoribosyl transferase (HPRT⁻) were used. These cells lack the scavenger pathway for the reutilization of hypoxanthine free base and cannot survive if an inhibitor, such as aminopterin, is used to block the de novo purine synthetic pathways. The myeloma parental cells may thus be selected against by culturing the fusion mixture in hypoxanthine/aminopterin/thymidine (HAT) medium, while the hybridoma cells will survive due to the contribution of HPRT by the antibody-producing fusion parental cells.

After a period of selection culturing, the surviving hybridoma cells may be cloned, stocks may be grown up by standard cell culture methods, and clones producing desired specific immunoglobulins may be detected by enzyme-linked immunosorbent assay (ELISA) or by other tests, based upon the use of the antigen against which the antibodies are directed.

The anti-LT-BNT antibodies obtainable through the use of this invention may further be used for the preparation of enterotoxin diagnostic tests. Such diagnostic systems could take the form of a radioimmunoassay, either in free solution or solid state. Alternatively, enzyme-linked immunosorbent assays could be produced, as could assays based on immunoblot analysis. These assays could be direct or indirect, with the application of a second antibody directed against the anti-LT-BNT antibodies. Numerous enzymatic activities could be coupled to the antibodies, with peroxidase, glucose oxidase, β-galactosidase and alkaline phosphatase being only a few of the possibilities. Those skilled in the art will also recognize that there are numerous other ways in which anti-LT-BNT antiserum could be utilized in a diagnostic capacity, such as in one of a number of agglutination tests. In such agglutination assays, the interaction of antibodies and any cholera-like enterotoxigenic bacterial enterotoxins (or binding subunits therefrom) may be detected using systems in which the particles are coated with the anti-LT-BNT antibodies. Such particles may be latex beads, liposomes, erythrocytes, polyacrylamide beads, or any of an number of suitable polymers.

6. EXAMPLE 6.1 GENERAL PROCEDURES FOR RECOMBINANT PLASMID PREPARATION

In the present invention, the source of the LT-BNT gene was E. coli 711 (10407), a K-12 transconjugant containing the LT-ST plasmid of a human enterotoxigenic isolate, H10407. This strain was derived by phenotypically tagging the enterotoxin plasmid of E. coli H10407 by transposition from an F'ts lac::Tn5 plasmid and conjugally transferring the Tn5-tagged plasmid to E. coli K-12 strain 711.

The plasmid was cleaved by a restriction enzyme to yield a small DNA fragment containing the LT gene. The DNA fragment was then ligated into a pBR322 plasmid, to produce a plasmid designated pDF82. E. coli K-12 transformants harboring the plasmid were then selected on the basis of antibiotic resistance markers. LT production by the transformants was established through the use of an enzyme-linked immunosorbent assay and an adrenal cell assay system.

The cloned LT-B DNA region was identified and then twice recloned, first into a pBR322 plasmid to give plasmid pDF87, and then into the M13-derived cloning vector pUC8. The resultant recombinant plasmid, pJC217, was cloned after transformation into E. coli K-12 and selection by antibiotic resistance and the loss of an enzymatic activity marker. LT-BNT recovered from pJC217, which was immunologically indistinguishable from pDF87 and native LT-B but completely non-toxic, was then isolated from host cell lysates for use as an immunogen.

A detailed description of each step in the construction follows.

6.1.1. CONDITIONS FOR RESTRICTION ENZYME DIGESTIONS

The restriction enzymes used were the products of Bethesda Research Laboratories, Inc., Gaithersburg, Md. A unit of enzyme activity is defined as the amount of enzyme required to completely digest 1.0 μg of lambda DNA in one hour at an appropriate temperature and in a 50 μl total reaction mixture volume.

Digestions were carried out by incubating 2 μg of DNA with 10 units of enzyme at 37° C. for 30 minutes in 20 μl of buffer Reactions were stopped by heating to 70° C. for 5 minutes, and the overall conditions produced one cleavage per vector plasmid DNA molecule For Pst I and Hind III, the buffer consisted of 50 mM Tris-HCl (pH 8.0), 10 mM MgCl₂ and 50 mM NaCl. Other reactions were carried out essentially as described by the manufacturer.

6.1.2. PURIFICATION OF DNA DIGESTION PRODUCTS

Following restriction enzyme treatment of pDF87, the digestion mixture was subjected to electrophoretic separation in vertical gel slabs containing 1.2% low melting point agarose in 40 mM Tris, 0.2M sodium acetate and 2 mM EDTA (pH 7.8). Electrophoresis was carried out at 10 volts per cm, and the slabs were then stained with ethidium bromide and visualized under ultraviolet light, as described by Bolivar and Backman [Methods in Enzymology 68:245 (1979)].

The separated DNA fragments were then excised from the gel, the gel was melted, and the LT-B DNA fragment was extracted with phenol.

6.1.3. T4 DNA LIGATION

Ligation reactions were carried out with T4 DNA ligase from Bethesda Research Laboratories, Inc., Gaithersburg, Md. A unit of T4 DNA ligase activity is defined as the amount required to catalyze the conversion of 1 nmole of ³² PPi into [α/β³² P]-ATP at 37° C. in 20 minutes.

DNA ligations were performed using 10 units of enzyme per μg of DNA at 4° C. for 18 hours. The buffer contained 66 mM Tris-HCl, 6.6 mM MgCl₂, 10 mM dithiothreitol and 66 μM ATP at pH 7.6.

To reduce recircularization, in some cases plasmid pBR322 was treated with alkaline phosphatase conjugated to Sepharose before ligation. The enzyme used was MATE-BAP from Bethesda Research Laboratories, Inc., Gaithersburg, Md. One unit of MATE-BAP is defined at the amount of enzyme that hydrolyzes 1 nmole of ATP in 30 min. at 37° C. The enzyme was used at a concentration of 500 units per μg of DNA at 65° C. for 1 hour in 10 mM Tris-HCl, pH 8.0. Following reaction, the enzyme was removed by centrifugal pelleting.

6.1.4. TRANSFORMATION AND ISOLATION OF RECOMBINANTS

The transformation of E. coli K-12 strains was carried out as described by Bolivar and Backman [Methods in Enzymology 68:245 (1979)]. Cells were made competent by incubation in 30 mM CaCl₂ at 0° C. for 20 minutes. Then 0.2 ml aliquots of 10× concentrated cells were added to DNA in 0.1 ml of cold ligation buffer supplemented with 30 mM CaCl₂ and incubated at 0° C. for 1 hour. The cells then heated to 37° C. for 2 minutes, held at room temperature for 10 minutes, and were diluted into 4 ml of Luria broth (L broth) Per liter, L broth contains 10 g of Bacto tryptone, 5 g of Bacto yeast extract, and 10 g of NaCl, all adjusted to pH 7.5 with 1M NaOH.

After 3 hours of incubation at 37° C., transformants were selected on Trypticase soy agar [BBL Microbiology Systems, Cockeysville, Md.] or YT plates, using appropriate antibiotics or enzymatic activity markers as described infra.

6.2 METHODS FOR LT GENE PRODUCT ANALYSIS

At each stage of the cloning procedure, the E. coli K-12 transformants were analyzed for the quality and quantity of LT or LT-B production by enzyme-linked immunosorbent assay (ELISA) To determine the toxicity of their gene products, the transformants were also analyzed by a mouse adrenal cell assay system in which cells exposed to enterotoxigenic E. coli or to their toxins exhibit readily detectable morphological changes.

6.2.1. ENZYME-LINKED IMMUNOSORBENT ASSAY (ELISA)

As described by Clements et al. [Infect. Immuno. 40:653 (1983)], clones to be analyzed were cultured overnight at 37° C. in 20 ml of Trypticase soy broth [BBL Microbiology Systems, Cockeysville, Md.], centrifuged, suspended in 2 ml of buffer containing 0.05M Tris, 0.001M EDTA, 0.003M sodium azide and 0.2M NaCl [pH 7.5], and disrupted by sonication with a Branson sonicator for 12 seconds at a power setting of 100-150 watts. The resultant lysates were clarified by centrifugation and serially diluted in pH 7.4 phosphate-buffered saline containing 0.05% Tween 20 (PBS-Tween) for analysis.

ELISA was carried out using two basic methods. In one method the wells of polystyrene microtiter plates [Costar, Cambridge, Mass.] were precoated with 50 μg per ml of type III gangliosides [Sigma Chemical Co., St. Louis, Mo.] to improve the binding of the LT-B subunit and, hence, to increase sensitivity. The microtiter wells were then filled with 0.2 ml aliquots containing the diluted lysate samples and incubated for 1 hour at room temperature. Following the incubation, the microtiter wells were emptied and washed three times with PBS-Tween. The wells were then treated successively for one hour each at room temperature with monospecific goat hyperimmune antiserum to LT [Clements et al., Infect. Immun. 29:91 (1980)] and with rabbit anti-goat antiserum conjugated to alkaline phosphatase [Miles Research Laboratories], with three PBS-Tween washings following each addition.

Alkaline phosphatase analysis was then performed by adding 200 μl aliquots of 1 mg/ml p-nitrophenyl phosphate substrate in 10 percent diethanolamine buffer (pH 9.8), incubating the plates for 60 minutes at room temperature, stopping the reactions by the addition of 25 μl aliquots of 3M NaOH, and measuring the results spectrophotometrically at 400 nm.

In some cases, a modification;.of the ELISA method of Holmgren and Svennerholm [Scand. J. Immunol. 8, Suppl. 7: 111-118 (1978)] was used instead. Microtiter plates were precoated with type III gangliosides, and 100 μl aliquots of samples to be tested in PBS with 0.5% gelatin (PBS-G) were pipetted into the microtiter wells. The plates were then incubated at 37° C. for 45 minutes, the wells were filled with PBS-G, incubation was continued for another 30 minutes at 37° C., and the plates were washed with PBS-Tween. The wells were then treated successively for 45-minute periods at 37° C. with 100 μl aliquots of PBS-G containing first antiserum to LT, and then antiserum directed against the anti-LT immunoglobulins that had been conjugated to horseradish peroxidase. Following each incubation period, the wells were washed three times with PBS-Tween.

Horseradish peroxidase analysis was then carried out with the use of o-phenylenediamine as a substrate The substrate was prepared immediately before use by dissolving 1 mg of o-phenylenediamine (Sigma Chemical Co., St. Louis, Mo.) per 1 ml of 0.1M sodium citrate buffer, pH 5.0. Then, an equal volume of a solution containing 1 ml of 0.3% H₂ O₂ per 50 ml of the citrate buffer was added to yield a final 0.006% H₂ O₂ concentration. Two hundred μl of the substrate was added to each well, the plates were incubated for 30 minutes at room temperature in the dark, and the peroxidase reaction was stopped by the addition to each well of 75 μl of 4M H₂ SO₄. The results were determined spectrophotometrically by measuring absorbance at 492 nm.

ELISA employing horseradish peroxidase was considerably more sensitive than that using alkaline phosphatase In other respects, however, the two systems were comparable.

6.2.2. Yl ADRENAL CELL ENTEROTOXIN ASSAY

Clarified cell lysates, prepared as described in Section 6.2.1. supra, were analyzed for toxicity in the mouse Yl adrenal cell system of Sack and Sack [Infect. Immun. 11: 334 (1975)]. Yl adrenal cells maintained in Ham's F10 medium with 12.5% horse serum, 2.5% fetal calf serum, and 40 μg per μl gentamycin were subcultured into 75 cm² culture dishes containing the same medium and incubated at 37° C. until the cells reached confluency.

Once the cells were confluent, the medium was replaced by fresh medium containing serially diluted aliquots of the E. coli LT-B clone lysates, and the cultures were incubated further. After 18 to 24 hours of incubation, the cultures were examined with a phase contrast inverted microscope for cell morphology. Under the influence of LT toxin, the normally flat adrenal cells become rounded. The sensitivity of the assay is such that as little as 0.2 μg of crude E. coli toxin purified LT per 200 μl of medium may be detected.

6.2.3. RAT ILEAL LOOP ASSAY OF ENTEROTOXIN ACTIVITY

Using the method of Klipstein and Engert [Infect. Immun. 23:592-599 (1979)], weanling Sprague-Dawley rats (Charles River Breeding Laboratories, Wilmington, Mass.) were surgically prepared by exposing the ileum and ligating 10-cm loops at the distal portion. Each animal was then challenged by direct inoculation with LT, LT-B or LT-BNT in 0.5 ml of sterile saline solution into the loop.

After 18 hours, the animals were sacrificed and the loops were examined for fluid accumulation. Data derived from the values from 5-8 rats at each enterotoxin concentration were expressed as fluid accumulation per centimeter of ileum. A positive response, denoted by a fluid accumulation of more than 50 μl/cm of ileum, was observed with as little as 1 μg of LT.

6.3. PREPARATION AND ISOLATION OF SPECIFIC LT-B-PRODUCING CLONES

Because of toxicity in the LT-B produced by the first clones, the LT-B gene was successively transferred into plasmid pBR322 and then into the M13mp7-derived pUC8 plasmid [Vieira and Messing, Gene 19:259 (1982)].

6.3.1. ISOLATION OF pDF82

The LT⁺ ST⁺ enterotoxin plasmid of human isolate H10407 was cleaved with the restriction I, to yield a 5.2 Kb DNA fragment (see FIG. 1). This fragment, which contained the LT gene, was then inserted into plasmid pBR322 which had been cut with Pst I and treated with alkaline phosphatase Ligation was carried out with T4 DNA ligase to produce a 10.4 Kb plasmid, designated pDF82. The ligation mixture was then used to transform E. coli MM294.

Plasmid pBR322 encodes both ampicillin and tetracycline resistance. When the plasmid is cut by the restriction enzyme Pst I and a DNA fragment is inserted, ampicillin resistance is lost, but not tetracycline resistance. These transformants were thus isolated by screening for ampicillin sensitivity (Ap^(s)) and tetracycline resistance (Tc^(r)) by growth, or inability to grow, in medium containing these antibiotics. After plating on Trypticase soy agar containing 25 μl per ml tetracycline, the cultures were incubated for 18 hours at 37° C. Growing colonies were then cloned in L broth, aliquots were spotted on Trypticase soy agar plates containing 100 μg per ml ampicillin and incubated for 18 hours at 37° C.

Ap^(s) Tc⁴ transformants were then assayed for LT production by the Yl adrenal cell system and ELISA. Plasmid DNA was isolated from several LT⁺ transformants by the method of Bolivar and Backman [Methods in Enzymology 68: 245 (1979)] and subjected to electrophoresis in 0.7% agarose. The conditions for electrophoresis and DNA visualization were as described in Section 6.1.2. above. One isolate, designated pDF82, was positive in both assay systems and showed only a single plasmid upon electrophoresis

When recut with Pst I, plasmid pDF82 yielded only two fragments that corresponded to the 4.3 Kb pBR322 cloning vector and the 5.2 Kb LT-encoding DNA fragment. Subsequent analysis of the recombinant plasmid with Pst I, Eco RI, Hind III, Hinc II, Hinf I and Ava II confirmed the size of the DNA fragment and the absence of internal Pst I sites.

6.3.2. ISOLATION OF pDF87

The cloned LT DNA region from pDF82 was recloned into the single Hind III site in the tetracycline resistance gene of plasmid pBR322 (see FIG. 2). Plasmids pDF82 and pBR322 were cut with Hind III, mixed and joined by T4 DNA ligase. The ligation mixture was again transformed into E. coli MM294, and transformants were selected on the basis of antibiotic resistance and sensitivity.

Because Hind III cleavage and DNA fragment insertion destroys tetracycline resistance but does not affect ampicillin resistance, Ap^(r) Tc_(s) cells were selected for. This was accomplished by a selection approach that was further enhanced by the use of cycloserine, which kills multiplying E. coli cells. After 18 hours of incubation in L broth with 50 μg/μl ampicillin, the culture was diluted 1:100 into fresh medium containing 4 μg per ml tetracycline. After 45 minutes of incubation, D-cycloserine was added to a concentration of 100 μg per ml, and incubation was continued for an additional 2 hours.

The culture was then centrifuged, and the pellet was resuspended in 20 ml of L broth. After 3 hours of further incubation, 0.1-ml aliquots were plated on Trypticase soy agar with 50 μg per ml ampicillin, and the resultant colonies were isolated. The transformants were then assayed for the production of LT-B by ELISA and for the absence of LT-A by lack of toxicity in the Yl adrenal cell assay. One clone largely meeting these requirements but retaining 1/1000 of the toxicity of LT from pDF82, on a weight basis, was designated pDF87. The reason for this toxicity was unclear, since no LT-A could be detected in pLF87 by SDS polyacrylamide gel electrophoresis, ELISA, or gel filtration under dissociating conditions.

Treatment of pDF87 with Hind III split the DNA into two fragments--pBR322 and a smaller (0.8 Kb) fragment coding for LT-B. Significantly, the 1.5 Kb Hind III gene fragment that codes for the production of LT-A was absent.

6.3.3. ISOLATION OF pJC217

The cloned LT-B DNA from pDF87 was recloned into the single Hind III site of cloning vector pUC8 (see FIG. 3). This vector, constructed by vieira and Messing [Gene 19: 259 (1982)], is derived from M13mp7. Plasmid pDF87 was cut with Hind III, and the 0.8 Kb LT-B DNA fragment was separated by electrophoresis in low melting point agarose (see section 6.1.2, supra) and extracted with phenol. pUC8 was then cleaved with Hind III, mixed with the purified LT-B gene fragment, ligated and transformed into an E. coli K-12. Insertion of DNA fragments at the Hind III site of PUC8 disrupts the structural gene for β-galactosidase activity which, together with unaltered ampicillin resistance in the plasmid, provides a basis for transformant selection.

Transformants were plated on YT plates (8 g Bacto Tryptone, 5 ng NaCl, 5 g Yeast Extract and 1.5 g agar per liter of water) containing 100 μg per ml of ampicillin and supplemented with 200 μg per ml of 5-bromo-4-chloro-3-indoyl-β-D-galactoside (X-gal) As described by Ruthere [Mol.Gen.Genet. 178:475 (1980)], Xgal is a β-galactosidase substrate that in the presence of the enzyme turns from colorless to blue. Following incubation for 18 hours at 37° C. on Xgal-YT, colorless colonies (whose plasmid-associated β-galactosidase activity had been insertionally inactivated) were isolated.

The Ap^(r) β-galactosidase⁻ transformants were then assayed by ELISA for LT-BNT production. LT-BNT from one positive clone, designated pJC217, was then tested Yl adrenal cells, where it was found to be completely non-toxic. The bacterium harboring this plasmid was designated E. coli strain JM83 (pJC217).

6.4. RECOVERY OF LT-BNT 6.4.1 GROWTH OF E. COLI K-12 CONTAINING THE pJC217 PLASMID

Subculturing of strain JM83 (pJC217) produces two types of colonies. One, designated type O, is small, raised and opaque in appearance. The other, designated type T, is larger, flat and translucent. It is important for the production of LT-BNT to utilize only type T colonies, since they produce 50-100 times as much LT-BNT as type O colonies. Because type O colonies can arise spontaneously from the T type, a single type T colony should be selected.

The choice of medium for the propagation of E. coli strain JM83 (pJC217) is not critical, and Trypticase soy broth, ML medium and the CAYE medium of Evans will suffice. In one embodiment of this invention, the organism transformed by plasmid pJC217 was streaked confluently on a 10×1.5 cm culture dish containing Trypticase soy agar with 100 μg/ml of ampicillin, to stabilize the plasmid. After incubation for 18 hours at 37° C., each plate contained a sufficient quantity of cells to serve as an innoculum for 10 liters of growth medium.

After the incubation, bacteria from the Trypticase soy agar plates were harvested with 5 ml of sterile 0.85 percent NaCl and used to inoculate CAYE medium with 0.5% glucose at a level of approximately 10⁶ CFU per ml. CAYE medium consists of 20 g of Casamino Acids, 6 g of Yeast Extract, 2.5 g of NaCl, 8.71 g of K₂ HPO₄ and 1 ml of trace salts (5% MgSO₄, 0.5% MnCl₂ and 0.5% FeCl₃) per liter of distilled water. Following inoculation, the cultures were incubated with agitation at 37° C. for 18-24 hours.

6.4.2. CELL DISRUPTION

Cells from the incubation medium were collected by centrifugation at 5,000 xg for 20 minutes at 4° C. The supernatant fluids were discarded into Clorox disinfectant, and the cell pellets were suspended in a minimal volume of buffer containing 0.05M Tris, 0.001M disodium EDTA, 0.003M sodium azide, and 0.2M NaCl, pH 7.5 (TEAN buffer). These cell suspensions were then pooled and stored frozen at -60° C. until needed.

The freezing of the cells generally provides sufficient disruption of E. coli K-12 for LT-BNT recovery. Mechanical disruptive techniques such as the use of sonication or a French press could be applied as long as scrupulous care was taken to avoid the generation of heat in the sample. Lysozyme should be avoided, since final LT-BNT recoveries with it as a lytic agent were decreased by 60 percent.

By swirling the storage containers for a minimum time in a 37° C. water bath, the frozen cell suspensions were rapidly thawed. The suspensions were then combined with an equal volume of TEAN buffer, swirled gently to mix, and then centrifuge at 5,000 xg for 20 minutes at 4° C. to sediment membranes and unlysed cells. Typical recovery of LT-BNT from a single application of this freeze-thaw procedure was 50 percent. Further recovery could be made by either repeating the cycle or by applying other disruptive techniques to the more refractory unlysed cells. All supernatant fractions from the centrifugation steps were then combined for further purification.

6.4.3. AFFINITY CHROMATOGRAPHIC PURIFICATION OF LT-BNT

The clear supernatant fluids from the centrifuged cell lysates were applied directly (not layered under buffer) to the top of a 2.5×80 cm Sepharose 4B [Pharmacia Fine Chemicals, Piscataway, N.J.] column which had previously been equilibrated in TEAN buffer at 4° C. The column was then purged with TEAN buffer at a flow rate of 20 ml per hour until effluent monitoring at 280 nm reached the baseline level. At that point, 0.2M galactose in TEAN buffer was applied and maintained at a flow rate of 20 ml per hour, while 6 ml fractions were collected. The emergence of LT-BNT was detected as a single peak of absorbance at 280 nm that slightly preceded the galactose front.

All operations were carried out at 4° C., and when complete, LT-BNT fractions were pooled, dialyzed against large volumes of TEAN buffer and lyophilized for storage.

6.5. ANALYSIS OF LT-BNT FOR TOXICITY

Samples of LT, LT-B from clone pDF87, and LT-BNT from clone pJC217 were analyzed in the Yl adrenal cell assay system as described supra, in Section 6.2.2. The protein content of the samples was determined by the method of Lowry et al. [J. Biol. Chem. 193:265-275 (1951)]. The results are shown in Table 1.

                  TABLE 1                                                          ______________________________________                                         Adrenal Cell Assay of Enterotoxin Activity                                     Preparation     Biological Activity*                                           ______________________________________                                         LT              0.038                                                          LT-B (pDF87)    39                                                             LT-BNT (pJC217) <25,000                                                        ______________________________________                                          *Activity is expressed as the nanogram quantity of each preparation            required to produce the rounding of 50% of the cells.                    

The data of Table 1 indicate that the toxicity of LT-BNT is drastically reduced than compared to that of LT, with LT being greater than 650,000 times more active LT-BNT. The results for LT-B show that it, while substantially less toxic than LT, retains considerable toxicity. This toxicity, which is comparable to that of the B subunit produced by the chromatographic separation of the complete LT enterotoxin into its subunits, renders ordinary LT-B unsuitable for use as a vaccine.

Analysis of LT and LT-BNT in the rat ileal loop assay, as described in section 6.2.3, also revealed the remarkable non-toxic character of LT-BNT. The results are shown in Table 2, wherein each value is the average from 5-8 rats.

                  TABLE 2                                                          ______________________________________                                         Rat Ileal Loop Assay of Enterotoxin Activity                                                         Fluid Accumulation*                                      Preparation                                                                               Amount (μg)                                                                            Ratio (μl/cm of ileum                                 ______________________________________                                         LT          1          65                                                      LT          10        421                                                      LT         100        818                                                      LT-BNT     100         0                                                       ______________________________________                                          *Fluid Accumulation Ratios greater than 50 are considered to be a positiv      result.                                                                  

As shown in Table 2, a 100 μg quantity of LT-BNT was completely inactive in the rat ileal loop assay. In contrast, the same quantity of LT produced a stimulation of ileal loop fluid accumulation that was more than 16 times the minimally significant level.

6.6. PREPARATION OF ANTISERUM AGAINST LT-BNT 6.6.1. PREPARATION OF ANTISERUM AGAINST LT-BNT IN MICE

Groups of 10 Balb/cj female mice (source), 4-6 weeks of age, were injected subcutaneously with 0.1 ml of saline solution (9 g NaCl/liter distilled water), either alone as a control or with a quantity of LT-BNT added. In some cases a booster injection was administered one week later. Immediately before vaccination and at varying times thereafter, blood specimens were obtained by tail snipping, and the serum was diluted and assayed for antibody activity by ELISA, as described supra, in Section 6.2.1. Antibody activity is expressed as the overall sample dilution needed to produce an absorbance falling in the linear region of the absorbance curve.

In a preliminary experiment, the effects of vaccination with varying quantities of LT-BNT were observed. The results are shown in Table 3.

                  TABLE 3                                                          ______________________________________                                         Dose Response Following Vaccination of Mice With LT-BNT                        LT-BNT        Antibody Activity                                                DOSE (μg)  Pre-Immune Post-Immune*                                          ______________________________________                                          --           .sup. ND.sup.+                                                                            ND                                                     1            ND         4                                                      10           ND         8                                                     100           ND         192                                                   ______________________________________                                          .sup.+ ND = Not Detectable                                                     *Post-immune activity was examined in tail snip blood samples one week         after vaccination with the quantity of LTBNT indicated, by alkaline            phosphatase ELISA. Antibody activity is expressed as the overall sample        dilution needed to produce an absorbance at 400 nm falling in the linear       region of the absorbance curve. Vaccination was carried out in saline          solution without adjuvant.                                               

As indicated in Table 3, antibodies against LT-BNT were not detectable in any of the animals prior to vaccination. By one week after vaccination, anti-LT-BNT antibodies could be detected. As expected, antibody activity increased with increasing exposure to LT-BNT.

To determine whether there could be a secondary antibody response to multiple vaccinations, groups of mice were injected with LT-BNT as described above, and then again one week later with a similar dose. The results are shown in Table 4.

                  TABLE 4                                                          ______________________________________                                         Effect of a Second Vaccination With LT-BNT on                                  Antibody Production in Mice                                                            Antibody Activity                                                      LT-BNT    Pre-     Post-1°                                                                           Second  Post-2°                            DOSE (μg)                                                                             Immune   Immune*   Vaccination                                                                            Immune*                                   ______________________________________                                         --        ND.sup.+ ND        -       ND                                        10        ND       8         +       7,144                                     10        ND       8         -         384                                     ______________________________________                                          .sup.+ ND = Not Detectable                                                     *Post-immune activity was examined in tail snip blood samples one week         after vaccination (1°) with LTBNT, by alkaline phosphatase ELISA.       At that time, either a second vaccination (2°) or saline solution       was administered, and postimmune activity was again examined one week          later. Vaccination was carried out in saline solution without adjuvant.  

The data in Table 4 shows that anti-LT-BNT activity could be detected only in animals that had been injected with LT-BNT. This activity increased markedly following a second challenge with LB-BNT. In contrast, when no LT-BNT booster was given, the increase in antibody titer, while substantial, was much lower

That the antibody activity produced in response to challenge by LT-BNT was persistant was shown in an experiment in which 10 Balb/cj female mice were injected with 10 μg of LT-BNT in saline solution without adjuvant. At various times thereafter, tail snip blood samples were analyzed for anti-LT-BNT antibody activity by horseradish peroxidase ELISA. As shown in Table 5, antibody activity reached a peak at two weeks after immunization but remained substantial even after 20 weeks. This strong and persistent response was seen even though no booster was administered and no adjuvant was used.

                  TABLE 5                                                          ______________________________________                                         Persistence of Antibody Activity in Mice                                       Following a Single LT-BNT Injection                                            Time After       Antibody*                                                     Vaccination (weeks)                                                                             Activity                                                      ______________________________________                                         0                ND.sup.+                                                      1                  320                                                         2                10,240                                                        4                5,120                                                         8                1,280                                                         20                 640                                                         ______________________________________                                          +ND = Not Detectable                                                           *Antibody activity is expressed as the degree of antiserum dilution neede      to produce a response in the linear region of the ELISA absorbance curve       at 492 nm.                                                               

6.6.2. PREPARATION OF ANTISERUM AGAINST LT-BNT IN A GOAT

Antiserum was produced in a 6 month old outbred goat by injecting 2 mg of purified LT-BNT subcutaneously, using standard methods. An identical booster vaccination was given 4 weeks later, and blood samples were harvested 10 weeks after the initial vaccination. Blood thus collected was allowed to clot, the clot was sedimented by centrifugation at 5,000 xg for 30 minutes, and the resulting serum was treated with 50% ammonium sulfate overnight at 4° C. The precipitate that formed was pelleted by centrifugation at 10,000 xg for 5 minutes, and the pellet was dissolved in 10 ml of TEAN buffer (Section 6.4.2), and dialyzed three times for a total of 24 hours at 4° C. against 200× volumes of the same buffer.

To render the resulting antiserum highly specific for LT-BNT, the preparation was twice purified by affinity chromatography. For this purification procedure, LT-BNT-Sepharose and CT-B-Sepharose were prepared by covalently binding purified LT-BNT or CT-B (List Biological Laboratories, Inc.) to cyanogen bromide-activated Sepharose 4B (Pharmacia Fine Chemicals, Piscataway, N.J.), according to the manufacturer's instructions. The dialyzed antiserum was first passed through a 1.5×18 cm column of LT-BNT-Sepharose, which was washed extensively with TEAN buffer. Immunoglobulins that had bound to the immobilized LT-BNT were then eluted from the column with 0.2M glycine-HCl buffer, pH 3.5, which was neutralized with 0.25M Tris as collected.

The eluted immunoglobulins were dialyzed against TEAN buffer as described, supra, and then applied to a 1×5 cm CT-B-Sepharose column. Immunoglobulins passing through the column were collected and were shown by ELISA to be specific for LT-BNT, but unreactive to CT-B.

6.6.3. PRODUCTION OF MONOCLONAL ANTIBODIES

To obtain spleen cells that can be made to produce antibodies against LT-BNT, 6-8 week old Balb/c mice (Jackson Laboratories) were sacrificed and the spleens were aseptically removed. Single cell suspensions were obtained by forcing the spleens through a wire mesh (Collector, E-C Apparatus Corp., St. Petersburg, Fla.). The splenocytes thus prepared were exposed to 1 μg/ml sterile LT-BNT in complete Dulbecco's Modified Eagle's Medium (DMEM) with 4,500 mg/liter glucose, 20% fetal bovine serum, 10% NCTC 109, 1% nonessential amino acids, 100 units/ml penicillin, 100 μg/ml streptomycin, 0.3 mM 8-bromoguanosine, 5×10⁻⁵ M 2-mercaptoethanol, and 50% thymocyte conditioned medium (TCM).

TCM, which is required for the successful in vitro immunization of spleen cells and which eliminates the need for feeder layers in cell cloning, was prepared by aseptically removing the thymuses from 4 to 6 week old BALB/c mice. The isolated thymuses were then disrupted as described above, the cells were cultured for three days in complete DMEM in a humidified 10% CO₂ incubator at 37° C., and the medium was harvested by pelleting the cells by centrifugation at 1,000 xg for 10 minutes and stored frozen at -20° C. until needed.

After 4 days of incubation in 10% CO₂ at 37° C., the LT-BNT-treated splenocytes were recovered by centrifugation at 1,000 xg for 10 minutes. To produce hybridomas, they were then mixed in a four-fold excess with murine myeloma cells (X63-Ag 8.653), and fusion was facilitated by the addition of 40% polyethylene glycol (PEG 1300, MCB Chemicals) and 5% dimethylsulfoxide Following one minute of gentle mixing at 25° C., the suspension was slowly diluted with DMEM, and the cells were removed from the medium by centrifugation.

The cells were then suspended in complete DMEM that had been supplemented with 5×10⁻⁵ M 2-mercaptoethanol, 30% TCM and HAT (10⁻⁴ M hypoxanthine, 10⁻⁵ M aminopterin, 3×10⁻⁵ M thymidine) at a dilution of 5×10⁵ myloma cells/ml. The cells were then cultured by distributing the suspension into 96-well tissue culture dishes, which were incubated in 10% CO₂ at 37° C. with twice weekly medium replacement. Visible colonies appeared in 30 to 60% of the wells after one week, and after three weeks, the supernatant medium in the wells was tested for the presence of LT-BNT-specific antibodies by horseradish peroxidase ELISA, as described in Section 6.2.1. Cells in wells whose medium was positive by ELISA were then cloned by limiting dilution or by plating in agarose.

Limiting dilution cloning was performed by diluting desired hybridoma cells to concentrations of 500, 50 and 5 cells/ml in complete DMEM supplemented with 40% TCM, and plating aliquots of the suspensions in the multiwell tissue culture dishes. Medium over the clones was then retested by ELISA after 2 to 3 weeks of incubation, and positive clones were expanded by subculturing in the same medium.

Cloning in agarose was carried out essentially as described by Coffino and Scharff [Proc. Natl. Acad. Sci. USA 68: 219-223 (1971)]. Tissue culture dishes (15×60 mm Corning) that had been coated with 4 ml of 0.4% Sea Plaque Agarose in DMEM were overlaid with 1 ml of 0.35% agarose containing 1000 cells/ml in complete DMEM supplemented with 40% TCM. The plates were incubated in a humidified 10% CO₂ incubator at 37° C. until the colonies reached an 8 to 16 cell size. Then, a 1 ml overlay containing 0.4% agarose in DMEM with a 1:50 dilution of anti-mouse IgM/IgG was added to each dish. After 2 to 3 days of incubation, visible precipitates marked the locations of immunoglobulin-secreting colonies, and the most vigorously secreting colonies were transferred by sterile pasteur pipette to 96-well plates. Especially strong anti-LT-BNT antibody producers were identified by ELISA, and stocks were developed through subculturing as desired. However derived, cloned cell stocks were preserved by storing frozen at -170° C. in 10% dimethyl sulfoxide with 90% fetal bovine serum.

To produce large quantities of monoclonal antibodies for use in diagnostic tests or for other purposes, the hybridoma cells were grown as ascites tumors. Eight week old BALB/c mice were primed with Pristane (0.5 ml/mouse, Aldrich Chemical Co., Milwaukee, Wis.), and then injected 10 days later with 10⁷ hybridoma cells. Ascites fluid, which developed in 7 to 10 days, was tapped with an 18 gauge syringe needle inserted into the abdomen of anesthetized animals. This fluid was then clarified by centrifugation at 2,000 xg for 10 minutes, preserved by the addition of 0.1% sodium azide and stored at 4° C.

6.7. NEUTRALIZATION OF ENTEROTOXIN ACTIVITY BY ANTISERUM AGAINST LT-BNT

To determine whether antiserum to LT-BNT could neutralize the activity of the enterotoxins of Vibrio cholerae and E. coli, quantities of these toxins that were 100 times the amount needed to produce cell rounding in the Yl adrenal cell system were incubated with various dilutions of goat antiserum (Section 6.6.2) for 1 hour at 37° C. Following the incubation, the samples were analyzed for toxicity in the adrenal cell system, as described in Section 6.2.2. As shown in Table 6, the diluted antiserum to LT-BNT completely neutralized both of the enterotoxins.

                  TABLE 6                                                          ______________________________________                                         Neutralization of the Activities of the Cholera and                            E. coli Enterotoxins in the Adrenal Cell Assay                                 by Antiserum to LT-BNT                                                                      Antiserum                                                         Enterotoxin.sup.+                                                                           Neutralization Titer*                                             ______________________________________                                         cholera toxin                                                                                 40                                                              E. coli LT   1,280                                                             ______________________________________                                          .sup.+ Enterotoxin used was approximately 100 minimal rounding doses.          *Titer is defined as the reciprocal of the highest serum dilution showing      complete neutralization of biological activity.                          

6.8. ANALYTICAL SYSTEMS BASED ON LT-BNT 6.8.1. ELISA ASSAY

In section 2.2.1, supra, a procedure is described for the ELISA detection of LT-BNT in samples. The ELISA system is equally applicable to the detection of specific antibodies to LT or LT-B in sera from human beings or from immunized animals. To use ELISA for antibody detection, 100 μl aliquots of 1 μg/ml LT-BNT in a coating buffer containing 50 mM sodium carbonate, pH 9.6, were pipetted into the wells of 96-well polyvinyl plates (Costar). The plates were incubated for 2 hours at room temperature, when the wells were filled with 0.5% gelatin in coating buffer and incubated overnight at 4° C. Any unattached antigen was then removed by three 3-minute washes with phosphate buffered saline containing 0.05% Tween 20 (PBS-T).

Samples containing antibodies to be analyzed, such as goat, human or mouse serum or murine hybridoma supernatant, were then diluted in PBS containing 0.5% gelatin and added 100 μl aliquots to the antigen-coated wells. The plates were incubated for 45 minutes at 37° C., the supernatant fluids were removed, and the wells were washed three times with PBS-T. Then, a second antibody (anti-goat, -human, or -mouse immunoglobulin antiserum as appropriate) coupled to horseradish peroxidase was added to the wells in 100 μl aliquots, and the plates were incubated for 45 minutes at 37° C.

After the second-antibody incubation, the plates were washed with PBS-T, and 200 μl aliquots of substrate (1 mg o-phenylenediamine in 1 ml of 0.1M sodium citrate buffer, pH 5.0, with 0.006% H₂ O₂) were added to the wells. After a 30 minute incubation at 25° C., the enzymatic reaction was stopped by the addition of 75 μl of 4M H₂ SO₄. The absorbances of the contents of the wells were then measured at 492 nm. Positive samples were taken to be those whose absorbance was at least twice that of controls, to which no primary antibodies had been added.

In a demonstration of the method, serum samples were collected from children and adults and analyzed for the presence of antibodies to LT-BNT, as shown in Table 7.

                  TABLE 7                                                          ______________________________________                                         Prevalence of Antibodies to LT-BNT In Human Serum                                                  Number   Positive                                          Subjects            Studied  Cases                                             ______________________________________                                         Children                                                                       18-24 months        10       2                                                 24-36 months        10       1                                                 36-48 months         9       2                                                 Adult                                                                          Pregnant Women      10       2                                                 General Population  10       5                                                 Pediatric Health Professionals                                                                      9       7                                                 ______________________________________                                    

The data in Table 7 are based upon the analysis of blood samples taken from individuals in the indicated age categories or from health care professionals working in the Division of Pediatric Infectious Diseases at The University of Rochester Medical Center. As might be expected for children with limited exposure histories, incidences of antibodies to LT-BNT were low. Incidences were somewhat higher for the general adult population, and higher still for the pediatric health professional group. The serum of 7 of 9 individuals from the latter group was positive, a finding not unexpected in view of the high probability of their contacting patients presenting enterotoxigenic diarrheal disease.

6.8.2. IMMUNOBLOT ANALYSIS

Mixtures of proteins containing LT or LT-B can be separated electrophoretically and the holotoxins or B subunits can be identified by the immunoblot method. As an illustration of the method, 10 μg of LT-BNT was subjected to stacking sodium dodecyl sulfate polyacrylamide gel electrophoresis by the method of Laemmli [Nature 227: 680-685 (1970)], using a 5% spacer and a 13% resolving gel.

Following electrophoresis, the gel was soaked for 45 minutes at 4° C. in an electroelution buffer containing 25 mM Tris-HCl, pH 8.3, with 192 mM glycine and 20% methanol. The proteins were transferred from the swollen gel to nitrocellulose paper (BA 85, Schleicher and Schuell) by electrophoresis in a Hoeffer Scientific Transphor electrotransfer unit, using the maximum amperage for two hours at 4° C.

To detect the LT-BNT, the nitrocellulose sheet was developed two ways. One section of the sheet was stained with Amido Black to reveal all transferred proteins. The remaining section of the sheet was soaked overnight at 4° C. in PBS buffer containing 0.1% sodium azide and 1% ovalbumin (PBS-Az-O) to block the remaining protein binding sites. The blocked nitrocellulose was then washed three times, for 10 minutes each, with PBS-T. Lanes containing LT-BNT were exposed for 2 hours at room temperature, either to affinity chromatographically purified goat anti-LT-BNT antiserum (Section 6.6.2) or to serum depleted of LT-BNT-binding antibodies by affinity chromatography. These serum preparations had been diluted 1-500 and 1-10⁶, respectively, with PBS-Az-O prior to use.

After the exposure to the sera was complete, the sheets were washed three times for 10 minutes each with PBS-T and then incubated for 1 hour at room temperature with horseradish peroxidase-linked anti-goat immunoglobulin, diluted 1,000-fold with PBS containing 1% ovalbumin. The sheets were again washed for three 10-minute periods with PBS-T, after which they were incubated with substrate (0.3 mg 3,3'-diaminobenzidine-HCl/ml of 50 mM Tris-HCl, pH 7.6, with 0.005% H₂ O₂) for 30 minutes at room temperature. Reaction was stopped by immersing the sheets in water and air drying them, and the locations of LT-BNT were indicated by brown bands

6.8.3. LATEX BEAD AGGLUTINATION

Latex beads coated with either antigen or antibody can be used to detect the corresponding specific antibody or antigen. To carry out the test to detect antibodies to LT-BNT, 0.8 μ latex beads (Difco Laboratories, Detroit, Mich.) were added to an equal volume of 1 μg/ ml purified LT-BNT in 0.1M glycine buffer, pH 8.2, with 0.15M NaCl. The suspension was incubated for two hours at 37° C. with gentle shaking, to ensure effective bead coating.

After the incubation, the beads were washed by gentle centrifugation for 12 minutes at 1000 xg. The supernatant fluid was replaced with PBS containing 0.1% bovine serum albumin, and Bromphenol Blue and sodium azide were added to the suspension, to final concentrations of 0.04. and 0.1%, respectively. To carry out the agglutination assay for anti-LT-BNT antibodies, 5 μl of the sensitized bead suspension was mixed with 15 μl of control buffer or a test sample diluted with PBS. Agglutination, which was read immediately after mixing, was defined as a change from a milky (control) suspension to a particulate suspension with a cleared background.

The agglutination assay was also applied to the detection of LT-BNT in samples by sensitizing the beads in an equal volume of the glycine buffer containing 0.25 mg/ml affinity purified goat antiserum or 0.1 mg/ml murine monoclonal antibodies to LT-BNT.

6.9. DEPOSIT OF MICROORGANISM

The LT-BNT-producing E. coli strain harboring plasmid PJC217 has been deposited with the Agricultural Research Culture Collection (NRRL), Peoria, Ill. and has been assigned the accession number NRRL B-15757. A culture of the deposited microorganism will be made available to the public upon the grant of a patent based upon the present application. The invention described and claimed herein is not to be limited in scope by the strain of microorganism deposited, since the deposited embodiment is intended as a single illustration of the invention. Any equivalent microorganisms that produce functionally equivalent enterotoxin subunits are within the scope of the invention. 

I claim:
 1. A substantially pure, non-toxic LT-B subunit of the E. coli heat labile enterotoxin which has the immunological characteristics of the expression product of the DNA encoding said subunit that is contained in plasmid pJC217.
 2. A vaccine formulation comprising the LT-B subunit of claim 1 and a compatible pharmaceutical carrier therefor. 